The present invention relates to hardening method and apparatus utilizing laser beams in which a laser beam irradiates a surface of a raw (or base) material.
When the laser beam irradiates the surface of the base material, a temperature of the localized surface layer of the material rises to a temperature suitable for heat treatment in a short time by a heat source of high energy density of the laser beam. When the heating is stopped, the heat is transmitted inside the material and then the material is rapidly quenched, thus performing a hardening treatment or working. A hardening technology utilizing the laser beam can provide such advantages as that localized hardening process can be made only to a necessary portion of the material, and in addition, oil, water or the like is not required for cooling and only a small strain due to the heat treatment is caused.
In prior art of certain technical field, a laser beam is utilized for irradiation to a raw material (which will be called merely “material” hereinafter) having a plurality of rows of portions to be hardened which are thermally influenced on each other. For example, as such material, there will be provided a track rail of a linear motion guide, and two rows of ball rolling surfaces extending in the longitudinal direction of the track rail are irradiated with the laser beams. The two rows of the ball rolling surfaces are formed closely, as shown in FIG. 9, in back-to-back close arrangement on both sides of a projected portion 1. In such formation, one of ball rolling surfaces 1a is first irradiated to carry out the first hardening process to thereby obtain a first-pass hardened layer 2a, and then, the other one of ball rolling surfaces 1b is irradiated to carry out the second hardening process to thereby obtain a second-pass hardened layer 2b. 
This hardening method requires two times of laser beam irradiation, thus requiring much time and labor. In addition to this defect, there may cause a case that the hardened layer 2a of the one of the ball rolling surfaces 1a which was once hardened (first time hardening) is again heated and softened (in a state as shown by a reference numeral 3 in FIG. 9) by the second time laser beam irradiation, or cause a case that a work distance in the second-pass is shifted by the strain caused at the first-pass time, which will result in deterioration in hardening (hardened) quality, thus also providing defects.